A weighing cell is a mechanical measurement transducer for the measurement of a mass, wherein the weight force exerted on the weighing cell by the weighing object is converted into an electrical signal. Examples are the strain gage weighing cell, the oscillating string weighing cell or the EMFR (electromagnetic force restoration) weighing cell. Weighing cells are often used in force-measuring devices, specifically in weighing scales, which convert the weight force of a load resting on the scale into an electrical signal.
In weighing cells which operate according to the principle of electromagnetic force compensation, the weight force of the weighing object is transmitted to an electromechanical measurement transducer either directly or by way of one or more fulcrum-supported force-transmitting levers. The measurement transducer produces a compensating force corresponding to the weight force of the weighing object and provides an electrical signal which is further processed and presented on a display by an electronic module of the processing unit.
An EMFR weighing cell includes a parallelogram linkage with a stationary parallel leg and with a movable parallel leg which serves as load receiver and is connected to the stationary leg by two parallel guides. In systems with a lever-reduction mechanism, a coupling element which is stiff in tension and flexible in bending is coupled to the load receiver and transmits the weight force to a balance beam whose fulcrum is supported by the stationary parallel leg. The purpose of this kind of weighing cell is to reduce the weight force of the weighing load to a magnitude where the measurement transducer is able to generate a compensating force and produce a measurement signal representative of the weight force. As is known in the art, the joints between the individual elements in weighing cells of high resolution are configured as flexure pivots. Flexure pivots define an axis of rotation between the two elements coupled by the pivot. In a weighing cell made of one integral piece of material, also referred to as a monolithic weighing cell, the flexure pivots can be realized in the form of thin material connections between the elements.
In EMFR weighing cells of a kind where the weight force is counteracted directly by the compensating force generated by the measurement transducer, i.e. without reduction by means of a lever system, the parallel guides are configured mostly as spring elements, elastic joints or diaphragm springs. In weighing cells of this kind, which are also referred to as direct-measuring systems, an individual measurement transducer opposes the weight force of the load with a compensating force of equal magnitude. If a plurality of measurement transducers is combined to counteract the weight force, each of them produces a corresponding partial compensation force.
In force-measuring devices of high resolution, the flexure pivots are thinner and therefore also more susceptible to suffer damage which can affect the weighing result or render the force-measuring device unusable. For example, if the weighing pan is hit or if the balance is dropped or set down abruptly, the parallel guides and other components can be stressed excessively. As a consequence, flexure pivots, elastic joints or diaphragm springs can become bent out of shape, cracked or even be destroyed.
A measurement transducer used in an EMFR weighing cell can for example be configured as a current-conducting coil in a permanent magnet. The coil is in most cases arranged on the balance beam, while the permanent magnet is attached to the stationary parallel leg. However, the reverse arrangement is also possible, where the permanent magnet is arranged on the balance beam and the coil on the stationary parallel leg. In the operating state of a force-measuring device, an electric current flows through the coil, whereby a compensating force is generated which counteracts the load place on the balance. A position-measuring device registers the deflection of the coil from its balanced position, whereupon a regulating unit regulates the magnitude of the current in response to the position measurement signal in such a way that the coil returns to its balanced position. When the coil is in its balanced position, i.e. when the sum of the forces acting on the system is equal to zero, the magnitude of the electric current is measured to determine the weighing result, which is then displayed.
An excessive amount of stress of the kind mentioned above can also affect the coil in the magnet system or the position-measuring device. In the production process of a force-measuring device, the step of adjusting the weighing cell is important for the sensitivity and accuracy of the device. The adjustment settings are valid only as long as the device is in the state in which the adjustment was performed. Under an excessive amount of stress, the alignment of the coil in the magnet system can change, or the position-sensing device can become dislocated relative to the balance beam, with the consequence that the weighing result is no longer correctly determined.
If the force-measuring device still delivers what appears to be a measurement of the weighing load in spite of damage to the weighing cell, be it in a flexure pivot or from a position change of the coil in the magnet system or of the position-measuring device relative to a position in which the force-measuring device was calibrated, the damage is not recognizable by currently available means. In spite of the damage, the force-measuring device delivers a weighing result, even though it is incorrect, as the force-measuring device does not appear to be functionally impaired, i.e. it seems to work error-free.
A method of monitoring and/or determining the condition of a force-measuring device is disclosed in EP 1 785 703 A1, wherein the force-measuring device includes a force-measuring cell that is set up in an interior space of a housing, as well as a sensor which measures climate-related parameters in the interior space which affect the operating lifetime of the force-measuring device. The method allows the condition of a force-measuring device to be monitored without the need to open the housing in order to determine the condition of the force-measuring device. However, this method has the disadvantage that a damaged condition of the weighing cell due to excessive stress, for example a damaged flexure pivot or a dislocation of the coil or the position-measuring device, cannot be detected.
To ensure that the force-measuring device is functioning correctly and that the user can have confidence in the displayed measurement value, the weighing cell has to be checked at regular intervals. This periodic inspection is in most cases performed by the manufacturer, which causes downtime of the force-measuring device at a cost to the user.
The objective of the present invention is to provide a method of verifying the functionality of a force-measuring device.
It should further be possible that the method can be performed at the work location of the force-measuring device and by the force-measuring device itself.